1. Field of the Invention
The present invention relates to an image processing apparatus for generating a high quality image from a low density original image received from a host computer by eliminating step-shaped jaggy lines and the like, and image forming apparatus using the image processing apparatus.
2. Description of the Related Art
In recent years, it is desired to output clear and sophisticated images. An output device such as a printer has been improved to print out high-resolution images. It is therefore required to output a high-density image from an original low-density image received from a host. For this purpose, technologies for enhancing image quality is widely applied in image processing equipment. Particularly, so-called smoothing function that eliminates step-shaped jagged lines included in low-density letters or line drawings has been introduced.
FIG. 19 shows a configuration of conventional image processing equipment. In FIG. 20, an illustrative chart of image processing is shown. As shown in FIG. 19, a window pattern having M×N dots is extracted from a binary image data expanded to a bitmap data to a window register 91 for collation. Characteristics of contours in a letter or a line drawing are extracted while this window pattern is collated to a predetermined collation pattern group 93. From the obtained characteristics in a contour, a dot converter 94 corrects a pixel of interest to enhance image quality on a dot bases. There is provided a pattern collation controller 92 for controlling pattern collation operation.
Thus, a 300 dpi image having step-shaped contours is converted into a 1200 dpi image having smooth contours by means of dot interpolation. In order to extract characteristics in this contour part, collation patterns are provided corresponding to respective contour patterns, facilitating to extract contour characteristics. According to this pattern collation method, however, it is required to prepare greater number of template (collation) patterns in order to obtain greater image enhancement effect.
To satisfy recent demand for higher quality images, it is required to output a smooth image by extracting characteristics of contour pattern very precisely. For example, contour characteristics in a circle as well as a slant line must be extracted. In conventional pattern collation method, it is required the number of collation patterns or collation circuits being as much as the number of contour patterns to be extracted. This requires large number of collation patterns or collation circuits to be prepared, which makes it difficult to realize low-cost image processing equipment for producing high quality images.
If, on the other hand, the number of collation patterns is restricted, the precise extraction of contour characteristics is degraded, which makes it difficult to obtain high quality images.